Olefins have been traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). There are numerous technologies available for producing oxygenates including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Syngas production processes are well known, and include conventional steam reforming, autothermal reforming, or a combination thereof.
Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor. The preferred methanol conversion process is generally referred to as a methanol-to-olefin(s) (MTO) process, where methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve catalyst.
Undesirable C4+ or C5+ olefins (heavy olefins) can be formed as byproducts of the MTO process. U.S. Pat. No. 5,714,662 to Vora et al. provides a practical use for a C3 and C4 olefin stream separated from an MTO product effluent and for water byproduct formed in the MTO process. More specifically, the Vora et al. patent is directed to a process for producing light olefins from crude methanol. The patent discloses that propylene and butylene fractions from the MTO product effluent can be converted to high octane ether and other high value products. Optionally, butylene from the MTO process can be dimerized and hydrogenated to produce a C8 alkylate having a high octane for use in blending motor gasoline.
Undesirable oxygenate compounds such as alcohols, aldehydes, ketones, esters, acids and ethers in the C1 to C6 range as well as trace quantities of aromatic compounds can also be formed in MTO reactors or in effluent processing. Additionally, a small amount of oxygenate from the feedstock, e.g., methanol or dimethyl ether (“DME”), can pass through the MTO reactor with the product effluent without being converted to desired product. As a result of oxygenate synthesis and/or oxygenate “pass through” in an MTO reactor system, the effluent from an MTO reactor can contain undesirably high concentrations of oxygenate compounds.
Oxygenate and heavy olefin compounds contained in an MTO product effluent can be undesirable for several reasons. For example, in order for an olefin-containing effluent to be suitable for polymerization the effluent should contain, at most, a relatively small amount of oxygenates. Thus, oxygenate separation from the desired light olefins can be required in order to provide an olefin product stream of a sufficient purity for polymerization. Increased investment is required to separate and recover the oxygenates from the desired light olefins.
Additionally, although the relative concentrations of oxygenates and heavy olefins in the product effluent are generally quite low, the total amount of oxygenates and heavy olefins in the effluent on an industrial scale can be enormous. A need exists for providing a use for the oxygenate and heavy olefin compounds which have been separated from the product effluent. That is, it is desirable to provide a practical use for the recovered oxygenate and heavy olefin contaminants so that commercial value can be realized from the oxygenate and heavy olefin contaminants separated from the product effluent.